It is conventional to provide harvesting machines such as forage harvesters with a metal detector for detecting tramp metal picked up from a field with crop material. A typical metal detector comprises means for generating a magnetic detection field through which the corp feed path extends, first and second detection coils disposed in the detection field, and first and second channels or detection circuits connected to the first and second detection coils, respectively. Tramp metal objects passing through the magnetic detection field disturb the detection field thus inducing an emf (electromotive force) across each detection coil. The emf induced across each coil is amplified and filtered by one of the detection circuits so as to produce two channel output signals which are each compared with a positive and a negative detection threshold value. If the magnitude of a channel output signal exceeds a threshold value, it is taken as an indication that tramp metal has been detected. The crop feed mechanism of the harvester is stopped so that the tramp metal is not transported into the cutter mechanism which chops the crop material.
The detection field, of necessity, is established in a region of the harvester having moving ferrous metal parts. Movement of the harvester parts in the detection field results in a noise component in the emf induced across the detection coils. Filters may be provided in the detection circuits to eliminate some, but not all, of the noise component. Therefore, it is conventional to set the detection thresholds at twice the average value of the noise component of each channel output signal so that the noise will not trigger a false indication that tramp metal has been detected in the crop feed path.
The noise generated by moving parts of the harvester varies. It is greatest at the time the crop feed mechanism is engaged and accelerating and drops to a lower value as the crop feed mechanism reaches its normal speed. The detection thresholds are therefore set at levels such as twice the start-up noise level to allow a safety factor. However, the sensitivity of the metal detector, that is, its ability to detect tramp metal objects of smaller size, decreases as the detection thresholds are increased. If it were not for the start-up noise the detection thresholds could be set at lower levels, that is, at twice the noise level at the normal operating speed of the crop feed, thus increasing the sensitivity of the detector without increasing the chances of triggering a false indication that tramp metal has been detected.
In my above-mentioned application I disclose a metal detector system wherein the positive and negative peaks of the noise component of a channel output signal are filtered to provide average positive peak and average negative peak values of the noise component. The adaptive detection thresholds are then set and varied according to the average peak noise values. The average noise values are stored and updated in a flash ROM (E.sup.2 PROM) so that they are available at system wake-up. This permits the setting of the adaptive detection thresholds and the filters at the levels existing at the time of system shut-down. Therefore, it is not necessary to wait for the filters to long-term (8 min) average the peak noise signals before the adaptive thresholds are set. This creates a problem in that the adaptive detection thresholds are set for the noise components existing under running conditions and do not take into account the larger noise components which exist at the time the crop feed mechanism is started.